Motile bacteria are attracted to certain chemicals and repelled by others; they exhibit chemotaxis. They possess a stimulus-response system in which the input is the local concentration of an attractant (repellent) and the output is the motion of the cell. What is the sequence of molecular interactions that couples input and output? It is known that some chemicals, e.g., sugars and amino acids, are sensed by specific receptors and that step-changes in net receptor occupancy lead to transient changes in the rotation of the flagella, but the structure and function of the machinery responsible for the integration, adaptation, and signal transmission is largely obscure. We will study the physiology of this system by examining the rotational behavior of Escherichia coli (wild type and mutant) tethered by their flagella and subjected to programmed stimuli (temporal changes in attractant or repellent) ranging in intensity from threshold to saturation and in duration from 10 to the minus 3rd power to 10 to the third power sec. We expect to learn whether the signal that controls the direction of rotation of the flagella is transmitted chemically or electrically, which rotational parameters are under its control, and how these parameters are influenced by adaptation. The work is intended to complement that on genetics and biochemistry pursued in other laboratories. The goal is an understanding of sensory transduction at the molecular level.